Nowadays, many consumer and mobile phone cameras offer some sort of high dynamic range (HDR) mode to deal with a high contrast in the subject. HDR, as its name implies, is a method that aims to add more dynamic range to digital images, where dynamic range is the ratio of bright to dark in an image. To achieve HDR with limited hardware capability, instead of just taking one picture, HDR uses several images, usually taken at different exposure times, or at different apertures, or different sensitivities (i.e. gains) and combines them into a single image.
However, the known methods of varying the exposure time introduce some limitations for controlling the image brightness. In particular, increasing the exposure time to obtain an HDR image can cause a drop in the frame rate. Further, if the known method of controlling the aperture is used, the image resolution will drop accordingly when the aperture size is reduced. Furthermore if the known method of increasing the sensitivity is used, chip noise can occur which results in brightness fluctuations on dark images.
From Document U.S. Pat. No. 6,753,876 B2, a method to increase the image dynamic range by using a laser for illumination and fusing multiple illumination levels of each pixel is known. It provides a floating-point illumination to achieve a high dynamic range image. This known method, however, has certain limitations. In particular, using a laser for the illumination measurement makes illumination systems much more complicated. Further, such systems are difficult to use for color imaging systems. Further, such systems are more sensitive to the surface finishing and surface facet angle which will limit the application dramatically. Further, the illumination of this known method uses a stripe pattern which will increase the difficulty of pixel fusion.
From Document U.S. Pat. No. 7,460,168 B2, an image processing apparatus for generating a wide dynamic range image is known. According to this known concept, a wide dynamic range image is generated in that a plurality of images is recorded under different exposure conditions. First, properly exposed areas and insufficiently exposed areas are divided (segmenting). In addition, a gradation of each image is performed. Then, a correction is performed to obtain corrected images. Finally, a single wide dynamic range picture is composed.
From Document US 2012/0257077 A1, an HDR image forming apparatus is known which also uses a similar technique in that the exposure time is controlled.
The above-mentioned prior art methods are typical examples of using exposure time control methods to obtain different intensity images and to fuse them into one wide dynamic range image. Indeed, the known methods of controlling the exposure time are technically simple and easy to use. However, the prior art techniques will introduce the above-mentioned limitations for controlling the image brightness. In particular, increasing the exposure time to obtain HDR images can cause a change or drop in the frame rate.